1. Field of the Invention
The present invention relates to a method for detecting saturation in a current transformer, and more particularly, to a method for determining whether or not the change of a secondary current generated in the current transformer is due to saturation of the current transformer.
2. Description of Related Art
A current transformer is a device that measures a current running in a current line and inputs the measured current to a protective relay in a power system. To maximize the flux linkage of the primary and secondary windings, iron-core current transformers are widely used.
The current transformer, as shown in FIG. 1, comprises an iron-core body (34), which concentrates magnetizing flux induced by a primary current running in a line (32), and a secondary coil (36), which coils the iron-core body in order to generate a secondary current from the flux induced in the iron-core body (34).
FIG. 2 shows the equivalent circuit of a normal current transformer, in which L is a magnetizing inductance of the current transformer; im is a magnetizing current; i2xe2x80x2 is a secondary current in accordance with the current transformation rate; and i2 is an actual measured secondary current. The magnetizing inductance L is not a constant value and varies depending on the amount of the current. When the flux increases and exceeds a predetermined value, the magnetizing inductance changes significantly. When there is inner change of the current transformer, the current transformer is considered to be saturated.
When the current transformer normally operates, the magnetizing current im is low, and the actual measured secondary current value is proportional to the primary current. However, when the current transformer is saturated and the magnetizing inductance value changes significantly, the secondary current value also changes significantly. This can be explained in the equivalent circuit of FIG. 2. As the value L reduces significantly, the magnetizing current im increases, resulting in the difference between i2 and i2xe2x80x2 currents. Accordingly, in cases where the current transformer is saturated, the relationship between the finally measured secondary current i2 and the primary current is different from that of normal operation. In addition, the saturation of the current transformer may cause malfunction or time delay of relays.
To detect saturation of the current transformer, which is most responsible for the malfunction of protective relays, the prior art has suggested a method for determining saturation by calculating the flux induced in the iron-core of the current transformer. However, this method is available only when no remanent flux exists in the core at the beginning. Otherwise, this method is difficult to apply, unless the initial value of the remanent flux is given. However, it is difficult to measure or assume the initial value of the remanent flux with the present technology, which is a critical defect of the prior art.
Unlike the prior art which requires information of the remanent flux to detect current transformer saturation, it is an object of the present invention to provide a method for determining saturation of a current transformer based on a predetermined decision function relating to the differences of secondary current values, regardless of remanent flux.
To achieve the above object, the present invention provides a method for determining saturation of a current transformer, comprising the steps of:
deciding a decision function which utilizes a secondary current value of the current transformer;
deciding a decision function value at each point of time by measuring the secondary current value at predetermined time intervals, and determining whether the decision function value is greater than a predetermined threshold value at each point of time; and
when the decision function value is determined to be greater than the predetermined threshold value, determining the beginning or end of the saturation, wherein the saturation of the current transformer is determined to end if the saturation of the current transformer begins before the point that the decision function value is greater than the predetermined threshold value, and the saturation of the current transformer is determined to begin if the saturation of the current transformer ends or does not begin before the point that the decision function value is greater than the predetermined threshold value.
At the step of deciding the decision function, the decision function is preferably one of a second-order difference, an adjusted second-order difference, an energy function of the second-order difference and an energy function of the adjusted second-order difference.
At the step of deciding the decision function, the decision function is preferably one of a third-order difference, an adjusted third-order difference, an energy function of the third-order difference and an energy function of the adjusted third-order difference.
At the step of deciding the decision function, the decision function is preferably one of a fourth order difference, an adjusted fourth order difference, an energy function of the fourth order difference and an energy function of the adjusted fourth order difference.
The step of determining the saturation includes a preventative malfunction step of determining one of the tiem points as an effective detection point and neglecting the other time points, wherein the decision function values are detected as being greater than a predetermined threshold value at a plurality of time points in series.